In lots of applications water or gas-tight seals can easily be formed by tightening two surfaces together. These often consist of crudely made and inexpertly assembled parts, yet they seal so perfectly that a molecule of air cannot pass through. At a fundamental or molecular level how does this work?
One example is threaded tapered pipe (e.g. NPT in the USA) which tightens to a form seal apparently as the thread tips deform under pressure. Threads can be cheaply made, rusty, etc. yet still work correctly.
Another example is when you tighten a flexible gasket between two metal pieces; the gasket deforms and makes the seal. Like a garden hose.